The new estimate highlights the huge difficulty facing the security services and border agencies in trying to keep Britain safe.

The Telegraph has been told that about “two dozen” individuals who have come back have been involved in plots in the UK – so far thwarted.

In recent months, the flow to Syria has “significantly slackened” with Islamist idealists, previously attracted to the region with a view to overthrowing the Syrian dictator Bashar al-Assad, deterred by the brutality and violence of the Isil leadership.

But more worrying for the UK authorities is the significant number of “hard core” extremists who have been attracted by the barbarity of the regime.

A far higher proportion of Britons who currently go to Syria, according to the Home Office analysis, are now intent on committing terrorist offences, attracted by the propaganda recruitment videos fronted by “Jihadi John”.

His unmasking as Mohammed Emwazi, a 26-year-old computer sciences graduate from the University of Westminster, highlights the difficulty facing the authorities in trying to keep track of​ UK-based jihadists. Emwazi, who was born in Kuwait but grew up in London, had been under surveillance by MI5 and on a no-fly watch list for about four years.

Ali Adorus, who grew up in west London, was sentenced to four and a half years in prison after being found guilty by the Ethiopian courts of attempting to establish Islamic rule through acts of terrorism

But in the spring of 2013, he escaped the UK undetected after slipping out of the country in the back of a lorry with the help of associates involved in a criminal network.

Another terror suspect, Ibrahim Magag, who had been placed under even tighter surveillance under a Terrorism Prevention and Investigation Measure, is now thought by some in the security world to have escaped with him.

Magag, 28, who was born in Somalia, jumped out of a cab at Euston Station on Boxing Day 2012 and then vanished. Intelligence services are understood to be investigating the possibility that Magag, who had attended terror training camps in Somalia and is accused of raising funds for al-Qaeda, might have been smuggled out of Britain either at the same time or using the same route as Emwazi.

The extent of Emwazi’s network is also now believed to be much bigger than previously thought, making it all the harder for the agencies to keep tabs on its various strands. Before joining Isil, Emwazi had previously attempted to join al-Shabaab, the al-Qaeda affiliated terrorist group operating in Somalia and East Africa, but had been turned back on landing in Tanzania following a tip-off from MI5 in May 2009.

The two other men travelling with Emwazi can now be identified as Ali Adorus, 33, a security guard, and a German Muslim convert named as Marcel Schrodl, who went by the name Omar or Umar. Schrodl’s current whereabouts are not known – potentially another headache for UK intelligence – but Adorus is presently languishing in a jail in Ethiopia.

Adorus, a father of two whose wife still lives in Finsbury Park in north London but who, like Emwazi, grew up in west London, was sentenced to four and a half years after being found guilty by the Ethiopian courts of attempting to establish Islamic rule through acts of terrorism.

The Telegraph can disclose that two other Britons were jailed with Adorus – Mohammed Ahmed and Ahmed Elmi. The trio had first entered Ethiopia in 2011 and were arrested in 2013 at a time when Emwazi was leaving London to fight his own jihad.

Emwazi is also connected to a terror cell operating out of west London that included the July 21 bombers who tried but failed to blow up the London Underground in 2005.

Emwazi and Adorus are further linked by connections to Cage, founded by Moazzam Begg, a former detainee at Guantanamo Bay, and which campaigns on behalf of Muslim prisoners and who are deemed “victims” of the war on terror.

Cage had highlighted the cases of both Emwazi and Adorus, accusing the UK authorities of harassing them and, in the case of Emwazi, pushing him over the edge and sending him to Syria.

Cage had until the events of the past 10 days been considered a reputable organisation, attracting endorsements from the likes of the actresses Joanna Lumley and Vanessa Redgrave.

On the Cage website, Miss Lumley says in a testimonial: “I join my prayers to yours for the safekeeping and early release of those detainees in Guantanamo, and my unending support for the cause of Cage prisoners.”

But Asim Qureshi, one of its directors, had praised Emwazi as a “gentle” man and blamed his plight on MI5. The charity’s accounts have been frozen and two significant donors told to stop funding it.

Emwazi’s radicalisation has been documented partly through Cage and other sources but the problem in stopping fanatics journeying to Syria was also highlighted in a court case last week involving a former teacher.

Jamshed Javeed, 30, had been a respected science teacher at Sharples High School in Bolton and regarded as a moderate Muslim. But he had become radicalised by the preachings of, among others, Anwar al-Awlaki, an al-Qaeda spiritual leader, so that in 2013 he had helped to pay for his brother Mohammed, then aged 20, to travel to Syria with two fellow students Khalil Raoufi, 20, and Raphael Hostey, 22. Raoufi was killed fighting for Isil last year and Javeed’s brother is also thought to have died.

Javeed himself was jailed for six years last week after being caught before he could leave the country and reach Syria. Javeed was only caught because his family hid his passport.

On the same day Javeed was being jailed, Abid Naseer was being found guilty of plotting to blow up the Arndale shopping centre in Javeed’s home city of Manchester.

Naseer, 28, from Pakistan, had entered the UK on a student visa and was arrested by British police as long ago as 2009.

But he was convicted in the US because the Crown Prosecution Service said it lacked the admissible evidence to secure a conviction in this country.

The case indicates the problems in pursuing terror suspects in the UK.

Those problems are not restricted to young men. The Metropolitan Police issued a humiliating apology to the parents of three schoolgirls who left their homes in Bethnal Green, east London, on Feb 17 before flying to Istanbul, and who are now believed to be in Syria.

Shamima Begum and Amira Abase, both 15, and Kadiza Sultana, 16, were reported missing to police by their families on the day of their disappearance. But it emerged that on Feb 5 police had given letters to the girls, addressed to their parents, requesting their daughters’ co-operation as part of a police inquiry into a school friend of the girls who had fled to Syria in December.

In a statement, Scotland Yard said: “We now understand that these letters were not passed on in every case. With the benefit of hindsight, we acknowledge that the letters could have been delivered direct to the parents.”